Examples of devices which vary the travel characteristics of a vehicle according to a driver operation amount comprise power steering systems which adjust a steering weight, an electronic control throttle of an engine of which a target opening can be varied according to an accelerator opening, an electronic control transmission which varies speed change characteristics, a traction control system which varies a traction of front and rear wheels, and a four-wheel steering system which varies the ratio of a rear wheel steering angle relative to a front wheel steering angle.
In these devices, the driver selects a preferred mode by suitably operating control switches provided in front of the driver's seat so as to manually vary travel characteristics according to road conditions, etc.
Due to diversification of travel preferences in recent years and to various kinds of innovation, these devices are diversifying so that the number of travel characteristics to be set is increasing, and as a result, the number of switches for setting travel characteristics is also continually increasing. It is very complicated for a driver to manipulate all these control switches, and as it is also difficult to operate control switches while vehicle is traveling, it often occurred that the superior functions of travel characteristic modifying devices were not fully utilized.
In order to avoid such a complication, a drive force controller which automatically controls an engine output of a vehicle according to road conditions obtained from a navigation device using a global positioning system (GPS) is disclosed, for example, in Tokkai Hei 5-180023 published by the Japanese Patent Office in 1993.
In this device, when for example the navigation detects that the vehicle is traveling along a narrow road or winding road, the engine output is decreased accordingly.
However if the drive force characteristics change according to the road conditions, the drive force of the vehicle may change abruptly and the speed will suddenly decrease or increase, even if the driver maintains a constant accelerator pedal depression amount.
When the vehicle is traveling in an urban area, the travel speed is relatively low, but a quick response is often necessary. In this case, the drive force may be insufficient due to decrease the engine output.
When road conditions improve, the drive force increases even in a traffic jam, but increase of drive force in a traffic jam actually makes driving more difficult.
Also, under conditions when a large drive force is required even if there are bends such as on a mountain road, decreasing the drive force leads to inadequate hill climbing ability.
Conditions such as these make the driver uncomfortable, and contribute to making driving more difficult.